The present invention relates to a sub-transmission for a vehicle, and more particularly to a planetary sub-transmission capable of shifting between high and low speed drive ranges by means of a planetary gear disposed between an input and an output shaft.
Various types of planetary secondary- or sub-transmissions are known in the art. One such planetary sub-transmission is disclosed, for example, in Japanese Utility Model Unexamined Publication No. 60-127,232 entitled "Power Transfer Apparatus for a Vehicle", laid open to the public Aug. 27, 1985, the disclosure of which is hereby incorporated by reference. In this planetary sub-transmission, shifting between drive ranges is effected by coupling coaxially disposed input and output shafts either directly or through a planetary gear unit having a sun gear and a ring gear, the sun gear and ring gear being fixed to the input shaft and to a transfer case, respectively. In this apparatus, the input and output shafts are coupled to each other directly for a high speed drive range, or indirectly, namely through the planetary gear unit, for a low speed drive range. For changing as between the direct and indirect coupling of the input and output shafts, a shifting means is operated to bring an input pinion gear of the output shaft selectively into engagement with an output pinion gear of the input shaft, or a carrier member of the planetary gear unit.
When direct coupling is established between the input and output shafts by means of the pinion gears fixedly mounted on the input and output shafts, the carrier member of the planetary gear unit is disengaged from the pinion gear on the output shaft and caused to idle or race. Such idling of the carrier member, arranged around the sun gear, and generally having a large moment of rotational inertia, tends to cause a large loss of driving torque between the input and output shafts particularly during high speed driving.